The present invention relates to a method of finding the image positions of three or more "fiducial markers", which are small objects that show up as small bright spots in images such as, for example, computed tomography (CT) and magnetic resonance (MR) images. This process of finding positions of fiducial markers in an image is also referred to as image space localization.
The images discussed above are "volume" images, meaning that they are sets of contiguous slices that span a volume of space. They are acquired by placing the patient into a CT scanner or an MR scanner. One or both of these imaging modalities or any other such imaging modality may be used for a given patient for a given surgery.
Different imaging modalities provide different types of information that can be combined to aid diagnosis and surgery. Bone, for example, is seen best on x-ray computed tomography (CT) images, while soft-tissue structures are best seen by magnetic resonance imaging (MRI). Because of the complementary nature of the information in these two imaging modalities, the registration of MR images of the head with CT images is of growing importance for diagnosis and for surgical planning. Furthermore, for the purpose of navigation during surgery it is helpful to be able to register images to the patient anatomy itself. Registration is defined herein as the determination of a one-to-one mapping between the coordinates in one space and those of another, such that points in the two spaces that correspond to the same anatomic point are mapped to each other.
U.S. Pat. Nos. 4,945,914, 4,991,579, 5,142,930, and 5,230,338 disclose a method for utilizing fiducial markers to establish a coordinate system that facilitates the registration of image spaces and physical spaces across time. The contents of U.S. Pat. Nos. 4,945,914, 4,991,579, 5,142,930 and 5,230,338 are incorporated herein by reference.
Briefly, these patents disclose using temporary or permanent markers that are imageable in the image space produced by a scanner. The markers may be attached to the skull via small posts that pierce the scalp and screw into the bone. The markers may also be attached by first drilling a hole and then inserting via a self-tapping thread a base into the hole, to which a temporary marker is subsequently attached. In any case, since the posts or bases are physically attached to the skull, the markers are referred to as "implanted" markers. Further, the markers are referred to as "external" markers, since the part of the marker that produces the bright spot in the image is outside the head.
In order to make full use of markers, they must be localized in the image space of the scan in question. Previous techniques have related to calling up successive images and manually locating spots whose brightness would appear to be indicative of the presence of a marker. However, this is an error prone, time consuming and labor intensive process. Therefore, there remains a need for the further development of more automated techniques for localizing markers in images such as MR and CT volume images.